1. Field of the Inventions
The present inventions relate to orthopedic joint replacements, and more specifically, to orthopedic devices that incorporate non-spherical articulating surfaces and a method of making and fitting such devices.
2. Description of the Related Art
Anatomically, a joint typically refers to a movable junction in the body of a subject between two or more bones. As used herein, the term is meant to include the different kinds of ligaments, tendons, cartilages, bursae, synovial membranes and bones comprising the mobile skeletal system of a subject in various quantities and configurations.
For example, the hip joint essentially is a ball and socket joint, linking the “ball” at the head of the thigh bone (femur) with the cup-shaped “socket” (acetabulum) in the pelvic bone. The ball normally is held in the socket by powerful ligaments that form a complete sleeve around the joint (the joint capsule). The joint capsule has a delicate lining (the synovium). Cartilage, which covers the head of the femur and lines the socket, cushions the joint, and allows the bones to move on each other with very little friction.
In a normal hip joint, the substantially spherical head of the thighbone (femur) moves inside the acetabulum of the pelvis. Normally, these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain, muscle weakness and less function, necessitating a total hip replacement. Several manufacturers make total hip replacement implants, which typically have three parts: the stem, which fits into the femur and provides stability; the ball, which replaces the spherical head of the femur; and the cup, which replaces the worn-out hip socket. Each part comes in various sizes in order to accommodate various body sizes and types. In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization in fit.
The shoulder joint is the body's most mobile joint, in that it can turn in many directions. The shoulder is a ball-and-socket joint made up of three bones: the upper arm bone (humerus), shoulder blade (scapula) and collarbone (clavicle). Two joints facilitate shoulder movement. The acromioclavicular (AC) joint joins one end of the collarbone with the shoulder blade; it is located between the acromion (the part of the scapula that forms the highest point of the shoulder) and the clavicle. The other end of the collarbone is joined with the breastbone (sternum) by the sternoclavicular joint. The glenohumeral joint, commonly called the shoulder joint, is a ball-and-socket type joint that helps move the shoulder forward and backward and allows the arm to rotate in a circular fashion or hinge out and up away from the body. The ball, or head, of the glenohumeral joint is the top, rounded portion of the humerus; the socket, or glenoid, is a dish-shaped part of the outer edge of the scapula into which the ball fits. The socket of the glenoid is surrounded by a soft-tissue ring of fibrocartilage (the glenoid labrum) that runs around the cavity of the scapula (glenoid cavity) in which the head of the humerus fits. The labrum deepens the glenoid cavity and effectively increases the surface of the shoulder joint, which helps stabilize the joint.
The bones of the shoulder are held in place by muscles, tendons (tough cords of tissue that attach the shoulder muscles to bone and assist the muscles in moving the shoulder) and ligaments (bands of fibrous tissue that connects bone to bone or cartilage to bone, supporting or strengthening a joint). A smooth, durable surface (the articular cartilage) on the head of the arm bone, and a thin lining (synovium) allows smooth motion of the shoulder joint. The joint capsule, a thin sheet of fibers that encircles the shoulder joint, allows a wide range of motion yet provides stability of the joint. The capsule is lined by a thin, smooth synovial membrane. The front of the joint capsule is anchored by three glenohumeral ligaments.
The rotator cuff, a structure composed of tendons and associated muscles that holds the ball at the top of the humerus in the glenoid socket, covers the shoulder joint and joint capsule. The rotator cuff provides mobility and strength to the shoulder joint. A sac-like membrane (bursa) between the rotator cuff and the shoulder blade cushions and helps lubricate the motion between these two structures.
The shoulder is an unstable joint easily subject to injury because of its range of motion, and because the ball of the humerus is larger than the glenoid that holds it. To remain stable, the shoulder must be anchored by its muscles, tendons and ligaments. Some shoulder problems arise from the disruption of these soft tissues due to injury or overuse, or underuse of the shoulder. Other problems can arise from degenerative processes.
For example, instability of the shoulder joint refers to situations that occur when one of the shoulder joints moves or is forced out of its normal position. The two basic forms of shoulder instability are subluxations and dislocations. A partial or incomplete dislocation of the shoulder joint (subluxation) means the head of the humerus is partially out of the socket (glenoid). A complete dislocation of the shoulder joint means that the head of the humerus is completely out of the socket. Anterior instability, for example, refers to a type of shoulder dislocation where the shoulder slips forward, meaning that the humerus moved forward and down out of its joint. Anterior instability may occur when the arm is placed in a throwing position. Both partial and complete dislocation cause pain and unsteadiness in the shoulder joint. Patients with repeat dislocation usually require surgery.
Bursitis or tendonitis can occur with overuse from repetitive activities, which cause rubbing or squeezing (impingement) of the rotator cuff under the acromion and in the acromioclavicular joint. Partial thickness rotator cuff tears, most often the result of heavy lifting or falls, can be associated with chronic inflammation and the development of spurs on the underside of the acromion or the AC joint. Full thickness rotator cuff tears most often are the result of impingement.
Osteoarthritis and rheumatoid arthritis can cause destruction of the shoulder joint and surrounding tissue and degeneration and tearing of the capsule or rotator cuff. In osteoarthritis, the articular surface of the joint wears thin. Rheumatoid arthritis is associated with chronic inflammation of the synovium lining, which can produce substances that eventually destroy the inner lining of the joint, including the articular surface.
Shoulder replacement is recommended for subjects with painful shoulders and limited motion. The treatment options are either replacement of the head of the humerus or replacement of the entire socket. However, available treatment options are less than adequate in restoring shoulder joint function. If the humeral or femoral head is to be replaced it is typically replaced by a resurfacing component, which replaces just the surface of the head or by a full head, which is typically fixed to a stem which is, in turn implanted into the humerus or femur for stability. When a reverse humerus is used, it is typically fixed to a humeral stem. For glenoid replacement a bearing surface is typically implanted directly into the scapula or fixed to a metal back, which is implanted into the scapula. A reverse glenoid replacement is typically fixed onto a metal back which is implanted into the scapula.
Most current shoulder and hip replacement components on the market consist of essentially spherical on spherical articulation. However, to achieve optimal results, different activities require different levels of constraint, joint thickness, soft tissue tension, moment, and arc of motion. Spherical articulation does not take this into account and therefore can not provide optimal results.
Exceptions to the typical spherical configurations that do exist have been limited to certain shoulder dual radius designs that incorporate either different inferior/superior and anterior/posterior radii, or different central and peripheral radii, both of which have been applied to both the glenoid and humeral head. While these dual radius designs may offer marginal improvements, they are still generalizations and do not account for the kinematic requirements of specific activities.